Abstract: The interaction between perchlorate and Ln(III) ions has been investigated in anhydrous acetonitrile for Ln = La, Pr, Sm, Gd, Dy, Ho, Tm and Yb. Control experiments have been carried out with Ln = Nd, Eu and Er. Vibrations assigned to unassociated (u), monodentate (m) and bidentate (b) perchlorate anions were identified in 0.05 M solutions of Ln(ClO4)3. Quantitative FT-IR measurements allowed the determination of the number of uncoordinated perchlorate ions, ClO4-(u), per Ln(III) ion, n(u), which increases from 1.43 (La) to 1.8-1.9 (Pr-Gd) and to 2.0-2.1 (Tb-Yb). Examination of other parameters such as the value and the variation of the nu-4(E)-nu-1(A1) and nu-8(B2)-nu-1(A1) splittings for ClO4-(m) and ClO4-(b) ions, respectively, the area of vibrational bands assigned to ClO4-(m), and the hypsochromic shift of vibrations assigned to bonded acetonitrile molecules, leads to the following interpretation. Both ClO4-(m) and ClO4-(b) ions are in the inner coordination sphere and the quantitative data point to a complicated situation in which several equilibria take place between species differing by the number of coordinated perchlorate ions, by the coordination mode of the perchlorate ions, and, probably, by the number of coordinated acetonitrile molecules. Changes in the average coordination number of the Ln(III) ions occur between La and Pr and at Gd. The equilibrium ratios for the formation of the monoperchlorato species could be evaluated for the heavier lanthanides: log K1 ranges between 1.8 and 2.7. Water molecules replace bonded perchlorate ions in the inner coordination sphere and so do chloride ions. The affinity of Ln(III) ions for chloride ions relative to perchlorate ions has been calculated to be log K(Cl/ClO4) = 1.1, 1.2 and 0.5 for La, Nd and Er, respectively. Finally, an estimate of the formation constant of the monochloro complex of Er yielded log K(Cl) = 2.6 +/- 0.7